I'm hosting a mail server for a couple different domains. Recently, the password of one of the users was guessed/hacked and the account was used to send out a couple hundred thousand messages before I noticed and was able to shut it down, which got the entire server blacklisted
I'd like to put a limit in qmail to rate-limit outgoing email on a per user/domain/time basis, but so far have not been able to find any built in functionality for that.
Any techniques you know of would be useful. One option I've come across is to send outbound email through SpamAssassin, but I'm concerned about blocking legit messages.